In general, heating and cooling apparatuses use a vapor compression system. The vapor compression system uses a gas medium such as chlorofluorocarbon and alternative chlorofluorocarbon to repeatedly exhaust heat by compressing the gas medium and absorb heat by expanding the gas medium, thereby producing heating and cooling effects. However, massive energy may be required at the compressing step, and the coefficient of performance (COP) can be as low as approximately 1.5. In addition, associated environmental problems are being pointed out because of the use of chlorofluorocarbon and alternative chlorofluorocarbon in the vapor compressing system.
In this context, in recent years, a technology for a magnetic refrigeration system is receiving increased attention as an alternative to the vapor compression system. The magnetic refrigeration system uses a magnetic material that exhibits a magnetocaloric effect. The magnetocaloric effect is a phenomenon in which, when a magnetic field environment changes because of application and removal of a magnetic field, the temperature of the magnetic material itself changes in association with the change of the magnetic field environment. In particular, the magnetic refrigeration system uses the magnetocaloric effect of the magnetic material to repeatedly exhaust heat by application (removal) of the magnetic field and absorb heat by removal (application) of the magnetic field, thereby producing heating and cooling effects. The magnetic refrigeration system has the advantage of having a COP of approximately 3 to 4 higher than that of the vapor compression system so as to ensure higher energy efficiency and is an environmentally-friendly system since chlorofluorocarbon or alternative chlorofluorocarbon is not used.
For example, Japanese Patent Unexamined Publication No. 2007-147209 discloses a magnetic refrigerator using a magnetic refrigeration system. In particular, the magnetic refrigerator includes a magnetic block including plural positive magnetic materials and negative magnetic materials which are alternately arranged, a magnetic field increasing-decreasing unit, and a heat switch. The positive magnetic materials produce heat when the magnetic field is applied and absorb heat when the magnetic field is removed. The negative magnetic materials absorb heat when the magnetic field is applied and produce heat when the magnetic field is removed. Hereinafter, a specific example of a system that achieves refrigerating effects (heating effects) according to the method described in Japanese Patent Unexamined Publication No. 2007-147209 will be explained.
First, a block is conceived to include magnetic materials having a configuration in which a negative magnetic material A, a positive magnetic material B, a negative magnetic material C and a positive magnetic material D are arranged in this order. The temperature change of the positive magnetic materials and the negative magnetic materials due to application and removal of the magnetic field is 5 degrees, and the initial temperature of the respective magnetic materials is supposed to be 25° C. When the magnetic field increasing-decreasing unit applies the magnetic field, the temperatures of the negative magnetic materials A and C decrease, and the temperatures of the positive magnetic materials B and D increase. Namely, the temperatures of A and C are 20° C., and the temperatures of B and D are 30° C. As a result, a temperature gradient is generated between the positive magnetic materials and the negative magnetic materials adjacent to each other.
Next, a heat switch is inserted between B and C. The heat is transmitted from B to C via the heat switch so that the temperature gradient between B and C disappears. On the other hand, the temperatures of A and D, which are not connected to other magnetic materials via the heat switch, are maintained by way of the heat insulation effect of an air layer. Namely, A is 20° C., B and C are 25° C., and D is 30° C. Subsequently, the heat switch between B and C is removed and the magnetic field is then removed by the magnetic field increasing-decreasing unit. The removal of the magnetic field increases the temperatures of the negative magnetic segments A and C and decreases the temperatures of the positive magnetic segments B and D. Namely, A is 25° C., B is 20° C., C is 30° C., and D is 25° C. Subsequently, the heat switches are inserted between A and B and between C and D. The heat is then transmitted between the magnetic materials connected via the heat switches so that the temperature gradients disappear. Namely, A and B are 22.5° C., and C and D are 27.5° C. Here, heat conduction between B and C does not occur because of the heat insulation effect of the air layer. As described above, by repeating the process including the steps of applying the magnetic field, inserting the heat switch, removing the magnetic field, and removing the heat switch, the temperature of A decreases and the temperature of D increases so that the temperature gradient between A and D increases. Accordingly, a cooling effect is obtained from A and a heating effect is obtained from D.
Here, the rotation rate of application and removal of a magnetic field per second is called the magnetic field frequency (the unit is Hz). Japanese Patent Unexamined Publication No. 2007-147209 teaches changing the conventional medium used for heat conduction from a liquid refrigerant to a solid heat switch. The change of the medium to the solid heat switch contributes to application and removal of the magnetic field at higher frequency in the magnetic refrigerator, which leads to a reduction in size of the apparatus. This is because solid heat conductivity is higher than liquid heat conductivity, and the time required for disappearance of the temperature gradient via the heat switch (the heat conduction between the magnetic materials) is shortened. In addition, the change of the heat conduction medium from the liquid refrigerant to the solid heat switch eliminates the need for a driving mechanism of the refrigerant, which helps to provide a low-cost magnetic refrigerator.